Showing posts from April, 2010

117 Mbps... But, Why?

It's no secret that 802.11n is a peculiar wireless technology. You've got multiple transmissions on a channel, a half dozen technologies and a few dozen data rates (at least) for the average AP to choose from. It can all make for some difficult troubleshooting, especially when looking at data rates. Here's a technique that I use to figure out which 802.11n technologies are missing (or not missing) when I'm trying to figure out why I'm getting a certain data rate.
Now, the first question one might ask when analyzing data rates is, what's the point? If I'm using my laptop at my desk and I have a 117 Mbps data rate, I'm not going to move to the break room just to get a bump to 130 Mbps. That is certainly true. But from a networker's perspective, you'd like to give your users the best experience possible, and there may be reasonable changes that would lead to better performance.
There are three questions that you can answer by looking at your data rat…

The iPad at Princeton: DHCP Problem/WiFi Problem?

The Wall Street Journal wrote today that the iPad is having problems on WLANs at the campuses of George Washington and Princeton. While the GW problem has yet to be described in detail, Princeton's Office of Information Technology has described their problems. The Princeton problem is about DHCP, and it reminded me of another problem that I saw with the iPad's WiFi behavior.

First, the background. reported that 20% of the iPad's seen on Princeton's WLAN had been banned for "malfunctions that can affect the entire school's computer system." Then the office of IT at Princeton clarified the issue by saying that iPads continue to use IP addresses after their DHCP lease time expires. This indicates that there is an error in Apple's implementation of DHCP.

The reason I decided to write about this is that there is an error in Apple's WiFi that has some relation to the DHCP problem that threatens Princeton's entire computer system (a tad overd…

Apple, I Love Ya But You're Shady (Another iPad WiFi Complaint)

If you're reading this post, there's a 100% chance that you have heard about the Apple iPad. There's also at least a 50% chance that you have heard about WiFi problems on the Apple iPad. And there's about a 10% chance that you've heard about the deceptive way that Apple markets the iPad's WiFi capabilities. That's what this post is all about.

Apple's technical specifications for the iPad say that dual-band (2.4 GHz and 5 GHz) 802.11n is supported. And it is. But what Apple doesn't tell you is that the version of 802.11n built in to the iPad is such that you get almost no speed improvement over 802.11g or 802.11a.

To understand what's going on with the iPad's mediocre 802.11n design, you have to understand a little bit about what goes in to 802.11n. There are three 802.11n improvements that bump up the maximum data rate from the 54 Mbps we had with 802.11a/g: more efficient (5/6 instead of 3/4) OFDM coding, double-wide (40 MHz instead of 20 MHz…